Since the news broke last year that the water supply in Flint, Michigan, is contaminated with lead, residents in other cities—like Seattle, Lake Mills, Wisconsin, and Newark, New Jersey—have discovered that their drinking water might also be tainted.

In the wake of these scandals, you might be wondering if you should be worried about your drinking water. It’s rational to be concerned: The effects of exposure to lead are very serious, especially in children, and include irreversible loss of IQ, learning disabilities, and even death.

To find out who should take action, we turned to the country’s top expert on contaminated water: Marc Edwards, a civil engineering professor at Virginia Tech who specializes in water treatment and corrosion. In the early 2000s, he exposed lead contamination in Washington DC.’s water supply and a subsequent cover-up at the CDC. Last year, he blew the whistle on the crisis in Flint after testing water samples and finding dangerous levels of lead. Now he’s heading up the committee that’s tasked with fixing the problem: engineering an upgrade to the Flint water main that will make the water safe to drink.

Figuring out exactly who should be worried about their water is complicated, but here is one upshot: Edwards says that if your home was built before 1986, you should get a lead filter. That’s because lead solder was routinely used to connect copper pipes in homes built before that year. “If you live in a high-risk home, you are better off buying a filter and being done with it.” Edwards says.

Why not test your water first? Home testing kits aren’t reliable, so you’ll have to have your water tested by experts, which costs around $30. And even if your water comes back clean, there’s no guarantee that it is actually safe: That’s because pieces of lead rust or lead solder flake off pipes into water from time to time, but you can’t know when that will be. “We used to tell people that if they tested their water once, it was safe. But now we know that’s not true,” Edwards says. “You can test the house four or five times, and it can look safe, and then the lead rust will fall off and can poison your child.”

For about the same price as one water test, you can simply buy a filter—be sure it is NSF certified for lead removal. Then you can be confident your water is safe. “If the filter is NSF certified for lead removal, that means it is subject to really rigid testing,” says Edwards. “Those filters are effective.”

Unfortunately, if your house was built after 1986, that doesn’t mean you’re completely in the clear. Another way lead gets into tap water is through lead service lines—the pipes that connect your home to the water supply. Some municipalities, like Chicago, installed lead service lines until 1986. New York City installed them until 1961. It completely depends on the city or town. So one step everyone should take is to call your water company and ask when lead service lines were discontinued. If your home was built before that date, you should err on the side of caution and get a filter. If your home was built after that date, you’re likely in the clear.

That is, if your water company is telling the truth. “Maybe your company kept good records and maybe they didn’t,” Edwards says. “Some people have been told they have lead service lines and they don’t, and vice versa. The only way to find out for sure if you have a lead service line is to dig a hole in the ground.”